1. Field of the Invention
The present invention relates to a mobile wireless communication system and a wireless communication apparatus that establishes wireless communication by controlling transmission power according to transmission power control information.
2. Description of the Related Art
A mobile wireless communication system that establishes wireless communication between wireless communication apparatuses such as a mobile station and a base station may have a configuration as is shown in FIG. 1. The mobile wireless communication system of FIG. 1 includes plural base stations 50a and 50b as wireless communication apparatuses that are located in different areas, and a mobile station 50c that conducts wireless communication while moving within the service areas of the base stations 50a and 50b. The base stations 50a and 50b and the mobile station 50c include antennas 51a, 51b, and 51c, transmission/reception units 53a, 53b, and 53c that include amplifiers, modulators, and demodulators, for example, and signal processing units 54a, 54b, and 54c, respectively. Also, the base stations 50a and 50b include interface units 52a and 52b, respectively, that form interfaces with a network (not shown).
The wireless communication scheme used in the wireless communication between the mobile station 50c and the base stations 50a and 50b may correspond to the W-CDMA (Wide-Code Division Multiple Access) scheme, the OFDM (Orthogonal Frequency Division Multiplexing) scheme, or other various schemes. Also, it is noted that transmission power control (TPC) information is transmitted from the base stations 50a and 50b to the mobile station 50c at predetermined intervals via downlink channels (dedicated channels) so that a signal transmitted from the mobile station 50c to the base stations 50a and 50b may be received with a suitable power that is sufficient for proper signal reception but is below a level that may cause interference with other channels. Based on the transmission power control information, the mobile station 50c conducts transmission power control to increase/decrease the transmission power of the transmission/reception unit 53c. The transmission power control information is generated according to the reception level (quality) of a signal such as a pilot signal that is transmitted from the mobile station 50c. For example, when the reception level (quality) is high, control information directing a decrease of the transmission power may be generated, and when the reception level (quality) is low, control information directing an increase of the transmission power may be generated.
In a mobile wireless communication system that realizes packet communication, an ACK signal is transmitted when a packet is properly received, and a NACK signal is transmitted when a packet cannot be properly received, the signals being transmitted via a dedicated channel. In such a system, even when packet transmission is rarely conducted, and transmission of reception response signals such as ACK and NACK signals is not necessary, a signal such as a pilot signal is continually transmitted via another dedicated channel for suitably adjusting the transmission power of the dedicated channel used for transmitting the reception response signals such as ACK and NACK signals. Thus, the mobile station unnecessarily consumes power for transmitting such signals.
In the W-CDMA scheme, HSDPA (High Speed Downlink Packet Access) for realizing a maximum downlink transmission speed of 14 Mbps is defined. According to this scheme, adaptive coding modulation is used for packet transmission. For example, QPSK (Quadrature Phase Shift Keying) modulation and 16-QAM (Quadrature Amplitude Modulation) may be adaptively interchanged to realize a transmission rate in accordance with the state of the wireless propagation path.
It is noted that HSDPA uses the H-ARQ (Hybrid Automatic Repeat Request) scheme. For example, in the mobile wireless communication system of FIG. 1, when the mobile station 50c receives packet data from the base station 50a and detects an error, the mobile station 50c sends a request to the base station 50a to retransmit the packet data. In turn, the base station 50a conducts retransmission of the packet data, and the mobile station 50c conducts an error correction decoding process using both the initially received data and the retransmitted data.
Also, it is noted that wireless channels such as HS-SCCH (High Speed-Shared Control Channel), HS-PDSCH (High Speed-Physical Downlink Shared Channel), and HS-DPCCH (High Speed-Dedicated Physical Control Channel) are used in HSDPA.
The wireless channels HS-SCCH and HS-PDSCH in a mobile wireless communication system correspond to downlink shared channels from the base station to the mobile station. The HS-SCCH corresponds to a control channel for transmitting parameter information pertaining to data transmitted by the HS-PDSCH. The parameter information may include modulation type information indicating the modulation method used in transmitting data via the HS-PDSCH, the diffusion code number, and pattern information for a rate matching process that is conducted on transmission data, for example.
The HS-DPCCH in the mobile wireless communication system corresponds to an uplink dedicated control channel from the mobile station to the base station, and is used upon transmitting from the mobile station to the base station an ACK signal or a NACK signal indicating whether data received via the HS-PDSCH are properly received. For example, in a case where CRC error is detected in the received data, a NACK signal is transmitted to the base station, and the base station conducts retransmission of the data in response to the NACK signal. Also, the HS-DPCCH is used to measure the reception quality (e.g., SIR: Signal to Interference Ratio) of a signal received from the base station, and periodically transmit the measurement result to the base station as a CQI (Channel Quality Indicator). The base station determines the state of the downlink wireless environment based on the CQI, and when it is determined that the wireless environment is in a good state, the base station switches to a modulation method that enables data transmission at a higher speed. On the other hand, when it is determined that the wireless environment is not in a good state, the base station switches to a modulation method that realizes data transmission at a lower speed.
FIG. 2 is a diagram showing the channels used in HSDPA. In this drawing, the wireless channels CPICH, P-CCPCH, HS-SCCH, HS-PDSCH, HS-DPCCH are shown. It is noted that the CPICH (Common Pilot Channel) and the P-CCPCH (Primary Common Control Physical Channel) correspond to downlink shared channels. The CPICH corresponds to a channel used for channel estimation and cell search at the mobile station, and as a timing reference of other downlink physical channels within the same cell; that is, the CPICH corresponds to a channel for transmitting the so-called pilot signal. The P-CCPCH corresponds to a channel for transmitting report information. The channels HS-SCCH, HS-PDSCH, and HS-DPCCH correspond to control channels as described above, and the HS-DPCCH is used to transmit the CQI and ACK/NACK signals.
In the illustrated example of FIG. 2, 15 slots make up one frame (10 ms), and since the CPICH is used as a timing reference, the beginning (head) of the frames of the channels P-CCPCH and HS-SCCH correspond to the beginning (head) of the frame of CPICH; however, the beginning (head) of the frame of the HS-PDSCH is delayed by two slots. Such a delay is made so that the mobile station may receive and identify information pertaining to demodulation of the HS-PDSCH beforehand. That is, information pertaining to the modulation method and diffusion code is provided beforehand via the HS-SCCH so that demodulation and decoding of the HS-PDSCH may be suitably realized. Also, it is noted that in the channels HS-SCCH and HS-PSDCH, three slots make up one sub frame.
According to 3GPP TS 25.212 v.5.7.0 (3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Multiplexing and Channel Coding (FDD)), the HS-SCCH represents the following information items:
(a) Xccs (Channelization Code Set Information); 7 bits; information on the diffusion code used for the HS-PDSCH;
(b) Xms (Modulation Scheme Information); 1 bit; modulation method used for the HS-PDSCH;
(c) Xtbs (Transport-Block Size Information); 6 bits; transmission data block size for error correction coding;
(d) Xhap (Hybrid-ARQ Process Information); 3 bits; process number for conducting retransmission control;
(e) Xrv (Redundancy and Constellation Version); 3 bits; parameter for rate matching;
(f) Xnd (New Data Indicator); 1 bit; information indicating whether data corresponds to new data; and
(g) Xue (UE Identity); 16 bits; user identification information.
As can be appreciated from the above descriptions, the HS-SCCH is made up of 37 bits, and by receiving the HS-SCCH, parameter information pertaining to the modulation method, the diffusion code, and error correction, for example, may be identified. In turn, demodulation and decoding of the HS-DSCH may be conducted according to the parameter information.
The information (a) Xccs represents information on the diffusion code used in transmitting data via the HS-PDSCH. For example, Xccs may represent a combination of the multi-code number and code offset. The information (b) Xms indicates the modulation method being used, and for example, Xms may correspond to ‘1’ or ‘0’ depending on whether the current modulation method corresponds to QPSK modulation or 16-QAM. The information (c) Xtbs corresponds to data for calculating the data size for transmitting a sub frame of the HS-PDSCH. The information (d) Xhap represents a process number for H-ARQ, and corresponds to a consecutive number with respect to that of a previously transmitted data block. It is noted that in the case of a retransmission process, the same process number assigned to the previously transmitted data block is used.
The information (e) Xrv represents redundancy parameter or constellation parameter information in the retransmission of the HS-PDSCH. It is noted that in some cases, the parameter information may be updated upon transmission or retransmission, and in other cases, the parameter information may be left as it is. The information (f) Xnd corresponds to data indicating whether the transmitted block corresponds to a new data block or a retransmitted data block. For example, in the case of a new block, ‘1’ and ‘0’ may be interchanged, and in the case of a retransmitted block, the arrangement of ‘1’ and ‘0’ may be left unchanged. The information (g) Xue corresponds to identification information of the mobile station (user).
By receiving the HS-SCCH, parameter information pertaining to the modulation method, the diffusion code, and error correction used at the HS-PDSCH may be identified, and demodulation and decoding of the HS-PDSCH may be conducted (e.g., see “Multi-Antenna Transceiver Techniques for 3G and Beyond” by Ari Hottinen, Olav Tirkkonen, and Risto Wichman; 3GPP TS 25.211, Physical Channels and Mapping of Transport Channels onto Physical Channels (FDD), v. 5.5.0; 3GPP TS 25.213, Spreading and Modulation (FDD), v. 5.5.0; and 3GPP TS 25.214, Physical Layer Procedures (FDD), v. 5.7.0).
Also, to enhance the W-CDMA system, use of the F-DPCH (Fractional Dedicated Physical Channel) is proposed (e.g. see 3GPP R1-031073). According to this technique, the same diffusion code is assigned to the pilot signals and transmission power control (TPC) information of a dedicated channel for plural users, and transmission is conducted through time division multiplexing within the same slot.
Generally, a mobile station (user) that receives data via a shared channel (traffic channel) such as the HS-PDSCH transmits traffic data via the HS-PDSCH, and thereby, data to be transmitted via the dedicated channel (e.g., DPCH as opposed to an HS channel) are virtually non-existent at such a mobile station. Nonetheless, TPC bits and pilot signals have to be transmitted in order to conduct transmission power control, and thereby, the dedicated channel (DPCH) and the HS-PDSCH have to be connected at the same time. In this case, a code is occupied even when data transmission does not have to be conducted, and when plural mobile stations (users) in similar situations exist, a deficiency in code resources may be created. In response to such a problem, the technique implementing the F-DPCH involves using the same code to conduct a diffusion process and time division multiplexing with respect to TPC bits and pilot signals of plural users that have no transmission data to be transmitted through a dedicated channel.
Also, as another measure for countering a deficit of code resources, Japanese Laid-Open Patent Publication No. 11-145901 discloses a wireless communication apparatus that conducts transmission power control through providing a dedicated power control channel, embedding TPC bits of plural users into the transmission power control channel through time division multiplexing, and embedding identical TPC bits into traffic channels for conducting packet transmission, wherein when a packet to be transmitted is generated, the TPC bits of the power control channel is converted into invalid information so that the TPC bits of the traffic channel is prioritized.
In the W-CDMA scheme, transmission power control (TPC) of a dedicated channel (DPCH) is conducted via an uplink channel. The transmission power control (TPC) involves controlling the transmission power of the mobile station by feeding back transmission control information referred to as TPC bits in slot units from the base station to the mobile station. On the other hand, in a packet communication scheme using shared channels such as the HS-PDSCH, pilot signals and TPC bits are periodically transmitted via uplink and downlink DPCH channels so that transmission power for transmitting a reception response signal is adjusted to a suitable level at a mobile station even when data are not addressed to the mobile station. In such a case, code resources have to be used for transmission power control of the respective dedicated channels, and thereby, a deficiency in code resources is created. Also, since the mobile station has to transmit pilot signals on a constant basis, a large amount of power is consumed as a result.
Also, a technique is proposed in the prior art that uses the F-DPCH in the W-CDMA scheme to integrate plural A-DPCH channels through time division multiplexing for realizing efficient use of code resources. However, since transmission power control (TPC) bits are transmitted in slot units, multiplexing may not be conducted for a large number of users. That is, according to this technique, a large number of mobile stations conducting packet transmission cannot be accommodated within the same area. Also, according to this technique, it is assumed that a signal is transmitted on a constant basis via the uplink channel. That is, the mobile station is arranged to transmit a signal such as a pilot signal for realizing transmission power control regardless of whether packet transmission is conducted. This arrangement is disadvantageous from the point of view of power conservation. It is noted that the mobile station uses a relatively small battery as a power source to conduct transmission/reception processes, and thereby, it is desired that power consumption be reduced in order to realize packet transmission for a long period of time.
Also, in the prior art technique involving the use of a power control channel and a traffic channel for transmitting TPC bits, when the TPC bits are transmitted via the traffic channel, the TPC bits at the power control channel have to be invalidated in order to prevent generation of a conflict. Such processes may be quite complicated thereby leading to high power consumption. Also, since the power control channel is arranged to be transmitted when a transmission control packet does not exist and is therefore not transmitted on a periodic basis, power control may not be stabilized.